The present invention relates to the field of image rendering by means of printing devices, particularly multicolour output devices; the invention especially concerns calibration of these devices.
A xe2x80x9ccolorantxe2x80x9d designates in this document an independent variable with which a printing device can be addressed. A xe2x80x9ccolorant valuexe2x80x9d, denoted as c, is an independent value that can be used to control a colorant of the printing device. The colorants of an offset printing press, for example, are the offset printing inks. It is customary to express the range of physically achievable values for the colorants of a device in %, which means that usually the colorant values range from c=0% to c=100%. In graphic arts, colorant values are often called dot percentages. A xe2x80x9ccolorant huexe2x80x9d is a basic colour of the printing device; the colorant hues of a traditional offset printing press and of a CMYK printer are cyan, magenta, yellow and black (as is customary, in this document C represents cyan, M represents magenta, Y represents yellow, K represents black and W represents white). The xe2x80x9chuexe2x80x9d of an object denotes whether its colour appears red, orange, yellow, green, blue, or purple (or some mixture of neighbouring pairs in this list). xe2x80x9cHuexe2x80x9d is also discussed under the xe2x80x98definition of remaining termsxe2x80x99 further below. A printing device with n colorants, wherein nxe2x89xa71, will also be called below a xe2x80x9cprinterxe2x80x9d or an xe2x80x9cn-ink processxe2x80x9d. A printing device with colorants of at least two different colorant hues is called a xe2x80x9cmulticolour output devicexe2x80x9d. An example of a multicolour output device is a CMY printer.
A xe2x80x9ccolorant spacexe2x80x9d is an n-dimensional space wherein n is the number of independent variables that are used to address the printer. In the case of an offset printing press, the dimension of the colorant space corresponds to the number of inks of the press.
A xe2x80x9ccolour spacexe2x80x9d is a space that represents a number of quantities of an object that characterise its colour. In most practical situations, colours will be represented in a 3-dimensional space that reflects some characteristics of the human visual system, such as CIE XYZ space (see xe2x80x9cThe Reproduction of Colour in Photography, Printing and Televisionxe2x80x9d by R. W. G. Hunt, Fountain Press, England, fourth edition, 1987, ISBN 0 85242 356 X, sections 8.4 and 8.5 for CIE XYZ; this book is referenced to below as [Hunt]). However, other characteristics can also be used, such as multispectral values that are determined by means of a set of colour filters; a typical example is an m-dimensional space of which the axes correspond to densities.
A xe2x80x9ccolorant gamutxe2x80x9d or xe2x80x9ccolorant domainxe2x80x9d is the delimited space in colorant space of the colorant combinations that are physically realisable by a given printer.
A xe2x80x9cprinter modelxe2x80x9d is a mathematical relation that expresses the printer""s output colour values as a function of the input colorant values for a given printer. The input colorant values are denoted as c1, c2, . . . , cn, wherein n is the dimension of the colorant space.
Because of the close relationship between an n-ink process and the printer model, the operations that are typical for a printer model are also defined for the corresponding n-ink process. The transformation of an n-ink process to colour space is equivalent to the transformation of the corresponding colorant domain to colour space by making use of the printer model.
A xe2x80x9ccolour gamutxe2x80x9d is the delimited region in colour space of the colours that are physically realisable by a given printer, while also taking into account possible extra limitations on colorant combinations. Take for example a CMY output device. A CMY process is a three-ink process. The colorant gamut 16 is a cube in the three-dimensional CMY colorant space 17, as shown in FIG. 1. The colorant combinations in this domain 16 are transformed to colour space 18 by the printer model. The range of this transformation is the colour gamut 19 of the three-ink process. This transformation is represented in FIG. 1.
A xe2x80x9cdensitometerxe2x80x9d is a photo-electric device that measures and computes how much of a known amount of light is reflected fromxe2x80x94or transmitted throughxe2x80x94an object, e.g. a receiving substrate such as paper or transparency film. A densitometer usually outputs a single value, i.e. a xe2x80x9cdensityxe2x80x9d. In most densitometers, a colour filter selected from a set of available filters can be put into the light path to limit the used light to the wavelengths that are relevant for the colour of which the density is to be measured (see e.g. xe2x80x9cOffsetdrucktechnikxe2x80x9d by Helmut Teschner, seventh edition, 1990, Fachschriften-Verlag, Fellbach, ISBN 3-921217-14-8, pages 542 to 549, for more information on densitometers and densities).
A xe2x80x9ccalorimeterxe2x80x9d is an optical measurement instrument that responds to colour in a manner similar to the human visual system (i.e. the human eye): a calorimeter measures the amounts of red, green and blue light reflected from an object, as seen by the human eye. The numeric values of the colour of the object are then determined in a colour space, such as the X, Y, Z values of the object""s colour in CIE XYZ space.
A xe2x80x9cspectrophotometerxe2x80x9d is an instrument that measures the characteristics of light reflected from or transmitted through an object, which is interpreted as spectral data. To compute spectral data, a spectrophotometer may examine a number of intervals along the wavelength axis, e.g. 31 intervals of 10 nm, and then may determine for each wavelength interval the reflectance (or transmission) intensity, i.e. what fraction of the light is reflected (or transmitted).
In general, colour is specified in a colour space that reflects some characteristics of the human visual system. Typical examples are CIE XYZ and CIELAB, but many more spaces exist such as appearance models (e.g. CIECAMs). In CIELAB space, a colour is represented by its three-dimensional co-ordinates (L*, a*, b*). Printers, however, cannot interpret colours specified in these spaces and hence conversions have to be made from such a space to the colorant space of the corresponding printer, e.g. from CIELAB space to CMYK space. This involves characterisation of the printer; see also FIG. 4.
xe2x80x9cCharacterisationxe2x80x9d of a printer is concerned with modelling the printer so as to predict the printer""s output colour values as a function of the input colorant values for the printer. The object of printer characterisation is not to change the device, but to describe how it works. Before a printer is characterised, it is first xe2x80x9ccalibratedxe2x80x9d, which means that the printer is put in a standard state; this is discussed below. Then, a characterisation target is printed by the printer. A characterisation target consists of a number of colour patches that are usually defined in the colorant space of the printer; a typical example of a characterisation target for a CMYK process is the IT8.7/3 target. To print a specific colour patch, the (CMYK) colorant values that correspond to the specific colour patch are used to address the printer. The colour patches of the printed characterisation target are then measured to determine their colour values, e.g. their colour co-ordinates L*, a* and b* in CIELAB space. A measuring device such as a colorimeter or a spectrophotometer may be used to determine these colour values. Based on the input colorant values, used to address the printer, and the measured corresponding output colour values, a printer model is created that predicts colour as a function of colorant values. This printer model is inverted to generate a xe2x80x9ccharacterisation transformationxe2x80x9d that transforms colours from colour space to colorant space. The characterisation transformation may be implemented e.g. as a xe2x80x9ccharacterisation functionxe2x80x9d, or as a xe2x80x9ccharacterisation tablexe2x80x9d supplemented with interpolation techniques. As an example, FIG. 4 shows a characterisation table 40 that transforms colours having co-ordinates (L*, a*, b*) in CIELAB space to colorant values cC, cM, cY, cK in CMYK colorant space. The creation of the characterisation transformation to obtain the desired colours on the output device is called xe2x80x9cdevice characterisationxe2x80x9d. Characterisation is also called xe2x80x9cprofilingxe2x80x9d, which means creating a file of data (a profile) that contains pairs of corresponding colour values and colorant values for the device. An often used profile is the ICC profile that meets the ICC standard; the ICC is the International Color Consortium.
If the colour values of a specific colour that is to be printed are known, e.g. the L*, a*, b* values in CIELAB space, the characterisation table can be used to calculate the colorant values that are to be used to address the printer in order to print the specific colour. However, to be able to reproduce colours in this way, the printer has to be kept in its standard state.
xe2x80x9cDevice calibrationxe2x80x9d is the action to put a device into its standard state; it involves running a check, preferably at regular points in time, to see whether the device has drifted away from its standard state. Device calibration especially applies to output devices, since they can be especially susceptible to drift: a monitor may lose brightness over time as phosphors fade; changes in room humidity or use of a fresh supply of toner or ink may cause a printer to produce different colour. The objective of device calibration, therefore, is to bring a device back to a known, standard state, so that it produces predictable colour every time it receives the same input colorant values. The present invention mainly deals with device calibration.
Current techniques for printer calibration use density measurements; usually the printing device prints a step wedge for each of its colorants; the patches of these wedges are then measured, e.g. by a densitometer, to determine the density of the patches. FIG. 2 shows an example for a CMYK printer that prints a cyan wedge 10, a magenta wedge 11, a yellow wedge 12 and a black wedge 13. Each wedge comprises one or more uniform patches, ranging e.g. from a colorant value c of 0% to 100% with steps of e.g. 5%; in this way, twenty-one patches per colorant are printed, such as patch 21 which is a cyan patch for c=50%. These wedges are the calibration target 15 upon which the calibration procedure is based. The colour characteristics of the printed wedges depend on the printer that is calibrated, and also on the type of receiving substrate and the types of xe2x80x9cmarking particlesxe2x80x9d, such as liquid ink drops, that are applied by the printing device to the receiving substrate. Therefore, when calibrating a printer, preferably always the same receiving substrate and the same marking particles are used. Two known methods are now described that use density measurements of the wedges.
In a first known method, each wedge is measured by means of a densitometer and the measured densities are compared to the reference densities that were measured initially, i.e. in the factory or when the printer came fresh from the factory. If the measured densities are different from the reference densities, the device has to be recalibrated. This is done by applying a one-dimensional lookup table (or LUT) per colorant before the colorant values c are sent to the printer. The LUT is filled out in such a way that the densities of the resulting step wedge are the reference densities. These one-dimensional lookup tables are also called calibration curves. FIG. 4 is an example in CMYK space showing LUT""s or calibration curves 45, 46, 47 and 48 that transform the colorant values cC, cM, cY, and cK, resulting from the characterisation table 40, into values tC, tM, tY, tK that are called xe2x80x9ctone valuesxe2x80x9d, denoted as t, in this document.
In a second known method, which is very common, density measurements are used to determine dot gain and the purpose of the calibration method is to keep the dot gain values equal to a given set of reference dot gain values. xe2x80x9cDot gainxe2x80x9d is the increase of halftone dots used in the printing process, due to the characteristics of marking particles, receiving substrate, printer. A first part of dot gain, called physical dot gain, is related to the spreading of the marking particles (e.g. ink) when a dot is printed onto the receiving substrate so that the density produced is greater than would be expected; some factors affecting dot gain are the thickness of the applied layer of marking particles, the physical properties of the marking particles (such as viscosity), the nature of the surface of the receiving substrate (such as whether it is glossy or matt). A second part of dot gain, called optical dot gain, is related to optical effects, especially light scattering. Optical dot gain depends on the properties of the receiving substrate. Suppose that the receiving substrate is paper; because of light scattering effects in the paper, a part of the incident light penetrates into the paper below the paper surface, and is reflected inside the paper, but the reflected light gets trapped under the dots and is absorbed by the underside of the dots. Therefore, the optically effective area of dots, printed on paper, is larger than the optically effective area of the same dots on a transparent material such as photographic film, when observed by means of the human visual system or when measured by a densitometer. A human observer will perceive a more intense colour on the paper than on the film and a larger density will be measured by a densitometer. Dot gain is important; if it is not under control, so that e.g. the printed dots of one or more colours are larger than desired, the printed image may exhibit a colour deviation.
Dot gain is defined as the difference between the assessed optically effective percent area coverage aeff of the printed dots and the colorant value c used to address the printer in order to print these dots:
dot gain=aeffxe2x88x92c
To assess the optically effective percent area coverage aeff, the Murray-Davies equation is commonly used:
aeff (in %) according to Murray-Davies             a      eff        ⁢          xe2x80x83        ⁢          (              in        ⁢                  xe2x80x83                ⁢        %            )        ⁢          xe2x80x83        ⁢    according    ⁢          xe2x80x83        ⁢    to    ⁢          xe2x80x83        ⁢    Murray    ⁢          -        ⁢    Davies    =      100    *                  1        -                  10                      -                          D              c                                                  1        -                  10                      -                          D              s                                          
with:
DC the density of the printed halftone dots;
DS the xe2x80x9csolid densityxe2x80x9d, i.e. the density of the printed 100% patch.
The Murray-Davies equation is based on the assumption that the optically effective percent area coverage aeff=AC/AS, wherein AC and AS represent the quantity of light absorbed by respectively the printed halftone dots and the printed 100% patch. Thus, the solid density DS is used as a reference with which the density DC of the printed halftone dots is compared.
Some densitometers provide an operating mode wherein they directly indicate the dot gain instead of the density; generally, dot gain is then calculated by means of the Murray-Davies equation.
It has been known that the Murray-Davies equation is not completely accurate; in fact, many factors are involved in dot gain, and it is very complicated or even impossible to model them. The Yule-Nielson equation proposes a correction factor for the Murray-Davies equation but is rarely used. Dot gain is often characterised by dot gain curves, that give dot gain as a function of the colorant value c. A dot gain curve may be determined by measuring densities and applying the Murray-Davies equation. FIG. 3 shows a dot gain curve 57 that is typical for offset printing systems: for a colorant value c of 50%, the dot gain is 15%. This means that the optically effective percent area coverage aeff of the printed dots will be aeff=50+15=65% if a colorant value of 50% is used to address the concerned offset press. Of course, a dot gain curve must be determined for each colorant of the printer; the dot gain curves also depend on the type of printer, receiving substrate, marking particles.
The purpose of the calibration method that uses dot gain is to keep the actual dot gain values equal to a given set of reference dot gain values. This may be accomplished as follows. For a specific colorant, the reference dot gain curve is given. This reference dot gain curve may be determined by printing and measuring a step wedge initially, i.e. in the factory or when the printer comes fresh from the factory. Often, however, the reference dot gain curves are defined and are not necessarily measured. The reference dot gain curves are typically curves with a dot gain of 15% at c=50%. The printing device now prints a calibration target, such as calibration target 15 shown in FIG. 2. The dot gain values of the calibration target are assessed, e.g. by measuring the densities of the step wedges in the calibration target, using the Murray-Davies equation and calculating the dot gain. The assessed dot gain values are compared with the reference values of the reference dot gain curves; if the assessed values are different from the reference values, a one-dimensional lookup table (or LUT) is applied per colorant before the colorant values c are sent to the printer. These one-dimensional lookup tables are also called calibration curves; FIG. 4 shows calibration curves 45, 46, 47, 48. The LUT is filled out in such a way that the dot gain values of the resulting step wedge are the reference dot gain values.
The above mentioned method to determine dot gain and the optically effective percent area coverage aeff has been used for quite some time for binary halftoning; it can be applied to multilevel halftoning techniques as well. In xe2x80x9cmultilevel halftoningxe2x80x9d, more than two different amounts of marking particles can be deposited by the printer onto the receiving substrate to form one microdot. A xe2x80x9cmicrodotxe2x80x9d is the smallest dot that can be addressed by the printer for application of a specific amount of marking particles; a halftone dot is composed of a number of microdots.
Dot gain, optical dot gain, the Murray-Davies equation, are discussed in xe2x80x9cOffsetdrucktechnikxe2x80x9d by Helmut Teschner, seventh edition, 1990, Fachschriften-Verlag, Fellbach, ISBN 3-921217-14-8, on pages 549 to 555.
FIG. 4 shows a Colour Management System (CMS) that comprises a characterisation table 40 and calibration curves 45, 46, 47 and 48. The purpose of a CMS is to provide colour consistency and predictability. In FIG. 4, the colour space is CIELAB and the colorant space is CMYK, but other spaces may be used, such as an RGB colorant space. The device independent CIELAB colour values (L*, a*, b*) are transformed by the characterisation table 40 to device dependent CMYK colorant values cC, cM, cY, cK, which are then corrected to tone values tC, tM, tY and tK by the calibration curves 45, 46, 47, 48. To print the characterisation target, preferably only calibration curves are used and no characterisation table is applied. To print the calibration target, preferably no characterisation table is applied and preferably no calibration curves are applied. A case wherein several calibration curves are applied subsequently is shown in FIGS. 4 to 5 (FIG. 5 is discussed in detail further below): tC1** is obtained from cc by applying successively calibration curve 45, ink mixing table 31, single ink calibration curve 42 and screening LUT 66. In such a case, LUT 66 may be filled out first, after which calibration curve 42 may be determined and then calibration curve 45. To determine calibration curve 45 in this case, preferably the calibration curve 45 itself, that is to be determined, is not applied in printing the calibration target (or curve 45 is replaced by a one-to-one LUT, i.e. by a unity transformation), and calibration curve 42 and LUT 66, that were determined previously, are preferably applied. The advantage of not applying the calibration curves, that are to be determined, in printing the calibration target, is that the new calibration curves for the device are directly determined from the device""s standard state and from measurements of the calibration target. This xe2x80x98directxe2x80x99 method is simpler and more accurate than the xe2x80x98indirectxe2x80x99 method that involves applying the xe2x80x98oldxe2x80x99 calibration curves when printing the calibration target. Applying the xe2x80x98oldxe2x80x99 calibration curves is more cumbersome: it requires measuring the printed calibration target and comparing the measurements to known reference values that represent the standard state of the device. If the comparison shows that the device has drifted away from its standard state, it has to be recalibrated. Recalibration may be done by printing a calibration target without applying the concerned calibration curves, as explained above. It may also be done by calculating the xe2x80x98newxe2x80x99 calibration curves from the measurements of the calibration target and from the xe2x80x98oldxe2x80x99 calibration curves, but this is less accurate than the direct method.
To calibrate a printer, densities will in general do a good job. However, it would be advantageous that, if some characteristics of the printing system were changed, the same calibration procedure and characterisation table could be used. In general, it is not possible to obtain the same colours on the receiving substrate, by using the same calibration technique and the same characterisation table, if a characteristic of the printing system has been changed. Moreover, quite a lot of printers are not stable at all and hence it is important to design a calibration system that is as robust as possible.
A first example is a CMYK offset printing system with two different types of screening. For a first set of images, conventional screening (Amplitude Modulation or AM screening) is used such as Agfa Balanced Screening(trademark) (ABS) at a line ruling of 150 lpi, and for a second set of images, Frequency Modulation (FM) screening is applied such as CristalRaster(trademark) (CR). The ABS technique is disclosed in U.S. Pat. No. 5,155,599, the CR technique is disclosed in U.S. Pat. No. 5,818,604. Both screening systems differ significantly. The dot gain e.g. in CristalRaster(trademark) is much higher than in ABS. Nevertheless we would like to use the same characterisation table, and, preferably, also the same calibration procedure for an image screened with ABS and for an image screened with CR, so that no new characterisation, and, preferably, no recalibration is necessary if a differently screened image is to be printed. As shown schematically in FIG. 5, screening dependent calibration curves or LUT""s 66, 67, 68, 69 may be used (screening dependent lookup tables are disclosed in EP-A-0 639 023). For example, screening algorithms 61 and 63 are ABS while 62 and 64 are CR. In FIG. 5, selector switches 71 and 72 are set to ABS; thus, in FIG. 5 the current image is screened using ABS. For an image screened in CR, selector switch 71 will be set to screening algorithm 62 and LUT 67 while switch 72 will be set to screening algorithm 64 and LUT 69. The LUT""s 66 and 68 are filled out in such a way that the dot gain values of the printed step wedge are the reference dot gain values when ABS is used, while LUT""s 67 and 69 are filled out in such a way that the dot gain values of the printed step wedge are the reference dot gain values when CR is used. In this way, the measured dot gain values will be the reference dot gain values, whether ABS is used or CR. However, there will be a significant difference from a visual point of view between an image screened with ABS and an image screened with CR, when calibration uses densities. This is due to the fact that the optical and physical dot gain, and the thickness of the ink layer in ABS and in CristalRaster(trademark) are different. Moreover, the result highly depends on the colour filter that is used in the densitometer to measure the densities. Therefore, an observer will see a significant difference between an image screened with ABS and an image screened with CR.
A second example is a printer for which the amount of ink, applied to the receiving substrate, changes over time for given colorant values. Suppose for example that the amounts of yellow and magenta inks that are really deposited on the receiving substrate increase over time for given colorant values. Thus, a printed 100% patch of yellow (cY=100) will look too yellow and a printed 100% patch of magenta (cM=100) will look too magenta. Using a calibration method that involves dot gain, as described above, will not solve this problem: a yellow 100% patch will remain too yellow, a magenta 100% patch will remain too magenta, the whole printed image will show a colour cast. In fact, a dot gain curve usually gives 0% dot gain for a colorant value c of 100% (see also FIG. 3) and the Murray-Davies equation is relative to the 100% patch, therefore, a calibration method using densities and dot gain does not work in this case.
A third example relates to printing devices, e.g. ink-jet printers, with two or more inks for the same colorant, e.g. two cyan inks. The main idea is to increase the visual resolution of the printer by attaining a higher resolution of optical densities (as opposed to spatial resolution). This is obtained by using inks that have the same colorant hue but a different colouring power and may be realised by using different concentrations of dyes or pigments in the inks. A multicolour output device, that uses a light and a dark cyan ink and a light and a dark magenta ink, can reproduce e.g. pastels at an apparent higher density resolution by making use of the light inks than if only the dark inks would be used. This is because the dots are more visible in light coloured areas when only dark inks are used, especially when using a screening technique such as error diffusion. Because the light inks barely increase the gamut of the devices, colorants which only differ in colouring power may be addressed by one single colorant value without a significant loss of colour gamut, e.g. light and dark cyan may be addressed by one value cC. This is obtained by means of so called ink mixing tables, that transform colorant values into different amounts of inks. In FIG. 5 an ink mixing table 31 is shown for a printer with light and dark cyan ink; the cyan tone value tC, which results from the cyan colorant value cC, see FIG. 4, is transformed by ink mixing table 31 into values tC1 and tC2 that ultimately determine respectively an amount of light cyan ink and an amount of dark cyan ink that is to be applied to a receiving substrate. Values tC1 and tC2 are preferably transformed by single ink calibration curves 42 and 43 into calibrated values tC1* and tC2*, after which screening dependent calibration LUT""s 66 to 69 and screening algorithms 61 to 64 may be applied as described above. Preferably, calibration is done in two steps: calibrating the single inks c1, c2, which corresponds in FIG. 5 to filling out LUT""s 42 and 43, and calibrating the xe2x80x98ink mixture(s)xe2x80x99, which corresponds in FIG. 4 to filling out LUT""s 45 to 48. In this way, a multicolour output device with two or more inks for the same colorant may be addressed e.g. by the traditional colorant values cC, cM, cY, and cK, but a colorant value c may correspond to a mixture of several inks. If in such a multicolour output device the calibration is performed by measuring densities and calculating dot gains from the measured densities, severe banding may occur, even in e.g. a cyan colour gradation. The cause is that in the Murray-Davies equation the densities DC and DS are compared to each other, i.e. two densities are compared that are obtained by means of different inks with different concentrations. The traditional calibration method using dot gain thus results in degraded image quality when applied to printing devices with two or more types of marking particles that have the same colorant hue.
It is therefore an object of the present invention to provide a calibration method and a system therefore that take into account changes of characteristics of the printing system.
It is an object of the present invention to provide a calibration method and a system therefore that are robust with respect to printer instability.
A xe2x80x9cquantityxe2x80x9d and a xe2x80x9cmagnitudexe2x80x9d, as referred to in the invention as claimed, are defined as follows. A xe2x80x9cquantityxe2x80x9d is the character of something that makes it possible to measure or number it or to determine that it is more or less than something else (see xe2x80x98Webster""s Third New International Dictionaryxe2x80x99, 1993). A xe2x80x9cmagnitudexe2x80x9d is the number or value of a quantity for a specific object. For example, if the length of a car is 4.5 m, then xe2x80x9clengthxe2x80x9d is the quantity, xe2x80x9c4.5xe2x80x9d is the magnitude and xe2x80x9cmxe2x80x9d (meter) is the unit.
It is generally agreed that colours have three main perceptual attributes: xe2x80x9chuexe2x80x9d, xe2x80x9ccolourfulnessxe2x80x9d and xe2x80x9cbrightnessxe2x80x9d; see also [Hunt], section 7.2.
As mentioned already above, xe2x80x9chuexe2x80x9d denotes whether the colour appears red, orange, yellow, green, blue, or purple (or some mixture of neighbouring pairs in this list).
xe2x80x9cColourfulnessxe2x80x9d denotes the extent to which the hue is apparent; colourfulness is thus zero for whites, greys and blacks (which are also referred to as neutral colours), colourfulness is low for pastel colours and is (normally) high for the colours of the spectrum. xe2x80x9cSaturationxe2x80x9d and xe2x80x9cchromaxe2x80x9d are terms that are related to colourfulness (see also [Hunt], section 7.2).
xe2x80x9cBrightnessxe2x80x9d denotes the extent to which an area appears to exhibit light; brightness is thus, usually: extremely high for the sun, very high for many other sources of light, high for whites and yellows, medium for greys and browns, and low for blacks. xe2x80x9cLightnessxe2x80x9d and xe2x80x9cluminancexe2x80x9d are terms that are related to brightness (see also [Hunt], section 7.2).
A xe2x80x9cpsychophysical quantityxe2x80x9d is a quantity PPQ that can be written as:   PPQ  =      T    ⁡          (                        ∑                      i            =            1                    N                ⁢                  xe2x80x83                ⁢                              W            i                    xc3x97                      Q            i                              )      
wherein:
T is a single transformation or a set of successive transformations; each transformation out of the set may be either a linear transformation (such as A*(xcexa3Wi*Qi)+B) or a non-linear transformation; T may also be the unity transformationxe2x80x94the output of the unity transformation equals the input, so that in this case PPQ=xcexa3Wi*Qi;
the sum xcexa3Wi*Qi includes one or more terms, i.e. Nxe2x89xa71;
each term of the sum is the product of a factor Wi and a factor Qi;
the weighting factors Wi depend on the sensitivity to colour of the human visual system;
the factors Qi are physical quantities or transformations of physical quantities;
the sum xcexa3Wi*Qi may also be an integral, such as ∫W(xcex)*Q(xcex)*dxcex wherein xcex represents wavelength. This definition is now illustrated by an example, without the intention to limit the definition thereto. Suppose that it is necessary to describe quantitatively the colour of a certain paint when viewed under illumination of a standard tungsten light source, that is, a tungsten lamp operating at a certain filament temperature. The first step is to measure the reflectance of the paint for wavelengths throughout the visible spectrum. This may be done by the use of a spectrophotometer that measures the reflectance e.g. every 10 nm at 31 wavelengths; the value of the reflectance at a given wavelength xcex is symbolised as Rxcex. The next step is to multiply Rxcex by the relative amount of light Lxcexxcex94xcex emitted at the same wavelength, wherein xcex94xcex is the length, e.g. 10 nm, of a wavelength interval. The product RxcexLxcexxcex94xcex describes the amount of light actually emerging from the paint in a wavelength interval xcex94xcex at wavelength xcex. Next, RxcexLxcexxcex94xcex is multiplied by the amount {overscore (x)}xcex at the same wavelength xcex of the first colour matching function {overscore (x)} for the CIE tristimulus values X, Y and Z (see [Hunt], page 105, FIG. 8.6). This product, {overscore (x)}xcexRxcexLxcexxcex94xcex, is summed over all wavelength intervals to obtain the quantity X=xcexa3{overscore (x)}xcexRxcexLxcexxcex94xcex. The same procedure is repeated with the second and third colour matching functions, {overscore (y)} and {overscore (z)}, to obtain respectively Y=xcexa3{overscore (y)}xcexRxcexLxcexxcex94xcex and Z=xcexa3{overscore (z)}xcexRxcexLxcexxcex94xcex. The numbers X, Y and Z are the desired result: they quantitatively describe the colour of the paint in the CIE XYZ space (see also the xe2x80x9cMcGraw-Hill Encyclopedia of Science and Technologyxe2x80x9d, 1977, ISBN 0-07-079590-8, under xe2x80x98Colorxe2x80x99). The quantity X is a psychophysical quantity: the weighting factors Wi are the amounts {overscore (x)}xcex of the colour matching function {overscore (x)}, and {overscore (x)} depends on the sensitivity to colour of the human visual system (see [Hunt], sections 8.4, 7.3 and 7.4) and the factors Qi are the physical quantities RxcexLxcexxcex94xcex. Similarly, Y and Z are psychophysical quantities.
A xe2x80x9cpsychovisual quantityxe2x80x9d PVQ is a psychophysical quantity for which differences of equal visual magnitudes for the human visual system correspond to substantially equal differences of the magnitude of PVQ. Let |q| represent the absolute value of q. Suppose that visual differences of a given magnitude, that are just noticeable by the human eye, correspond to differences in PVQ of which (xcex94PVQ)max=|PVQBxe2x88x92PVQA| is the largest and (xcex94PVQ)min=|PVQDxe2x88x92PVQC| is the smallest difference (see also [Hunt], section 8.6: xe2x80x9cUniform chromaticity diagramsxe2x80x9d and section 8.8: xe2x80x9cUniform colour spacesxe2x80x9d). PVQ is a psychovisual quantity if (xcex94PVQ)max less than R*(xcex94PVQ)min, with R less than 10, preferably R less than 5, more preferably R less than 3, even more preferably R less than 2 and most preferably R less than 1.5. xe2x80x9cHigh quality psychovisual quantitiesxe2x80x9d have a low value of R. For example CIE lightness L* and CIE chroma C* are high quality psychovisual quantities. The psychophysical quantity Y, from the CIE XYZ system, indicates brightness but it does not represent a uniform visual scale: to a human observer, the apparent difference between e.g. two samples of Y=10 and Y=15 is much greater than that between two samples of Y=70 and Y=75. On the contrary, CIE lightness, L*, is a psychovisual quantity: equal increments on the L* scale do represent approximately equal steps in the perceived lightness of related colours.
A xe2x80x9ccolour distancexe2x80x9d is a Euclidean or non-Euclidean distance in a colour space, which has a dimension mxe2x89xa71, or in a subspace of the colour space. A colour distance between two patches of a wedge is the colour distance between the two points that correspond, in the concerned colour space or subspace, to the two patches (e.g. a point with co-ordinates X1, Y1, Z1 corresponds in CIE XYZ space to a patch that has X1, Y1, Z1 colour values in CIE XYZ space).
A xe2x80x9cmarking particlexe2x80x9d is a particle that is applied to a receiving substrate by the printing device. The printing device may apply different types of marking particles to the receiving substrate. Marking particles of a specific type may contain a specific material such as a dye or a pigment to give the marking particles a specific colour; the colour of marking particles is called in this document the xe2x80x9ccolorant huexe2x80x9d of the marking particles. Different types of marking particles may have the same colorant hue; e.g. light cyan marking particles and dark cyan particles both have the same colorant hue, namely cyan. In ink-jet printing, the marking particles are liquid ink drops. In thermal transfer printing and in laser induced melt transfer printing, the marking particles are usually melted wax ink particles. In electro(stato)graphic printing, the marking particles are toner particles; liquid electrostatographic development (using a dispersion of solid toner particles in a dielectric liquid) as well as dry electrostatographic developers may be used. The colorant values of marking particles, when applied to a receiving substrate (e.g. fused to the receiving substrate in case of toner particles), may be different from the colorant values of the original marking particles; therefore, the colorant values referred to are those of the marking particles appearing on the receiving substrate.
A xe2x80x9creceiving substratexe2x80x9d may be a separate sheet or it may be a continuous web; it may be made of paper, of polyethylene coated paper, of plastic, of white poly(ethylene terephtalate), it may be a laminate of paper and plastic; it may be transparent or opaque; several kinds of receiving substrate are described in patent application EP-A-768 577. Measurements concerning colour have a different nature if the receiving substrate is opaque or transparent; e.g. for densities, on an opaque receiving substrate the measured densities are reflection densities, while transmission densities are measured on a transparent receiving substrate.
A xe2x80x9ccolour proofxe2x80x9d of a colour image is required for inspection and approval by the printer or his client before the colour image is printed on the production printing press. To make a traditional off-press proof, colour separations of the image (e.g. a cyan, a magenta, a yellow and a black separation) are produced on a substrate which may be a black and white photographic film; these colour separations are then used to make the colour proof of the image.
In xe2x80x9cdigital colour proofingxe2x80x9d, the digital data that represent the colour image are directly sent to a high resolution, high-quality printer that prints the colour proof on a receiving substrate, thus omitting the intermediate step of producing colour separations on a substrate.
The above mentioned objects are realised by a method and a system in accordance with the present invention as claimed in the independent claims. The dependent claims set out preferred embodiments of the invention.
As will become apparent from the following description and drawings, some of the disclosed embodiments do not require all the features of the invention as claimed in the independent claims; some of these disclosed embodiments may be the subject of a divisional application of the present patent application.
Preferably, the printed wedges are step wedges, that include a number of adjacent patches. However, a printed wedge may also be simply a set of non-adjacent patches or a wedge may consist of only a single patch. The surface area of each patch may be e.g. 15 mmxc3x9715 mm, but it may also be much smaller, or larger. The surface area should be large enough so that the magnitude of the first quantity that is determined is representative for the concerned patch. For example, for a patch that has a colorant value of 2% and that is printed using an error diffusion screening method, the surface area of the patch should be large enough to have a correct proportion of the (small) printed area to the (large) non-printed area; thus, the minimum required surface area of a 2% patch may be larger than the minimum required surface area of a 100% patch.
A magnitude of the first quantity q1 is determined for at least one and preferably for each patch of the first wedge, i.e. a magnitude m1A is determined for patch A of the first wedge, a magnitude m1B for patch B, m1C for patch C, etc. The first quantity q1 is a psychophysical quantity, preferably a psychovisual quantity, more preferably a high quality psychovisual quantity. Moreover, the first quantity is preferably determined with respect to a reference value which is the magnitude of q1 for the receiving substrate itself and which is, for paper, the so-called xe2x80x98white of the paperxe2x80x99 (see also [Hunt], page 116, the xe2x80x98reference white being usedxe2x80x99). A magnitude of q1 may also be determined for just one patch; in this case, preferably a magnitude of q1 is also determined for the receiving substrate itself in an area where no marking particles are applied.
In a preferred embodiment, the first quantity q1 is used in determining a magnitude m1AB=d1(A,B) of a colour distance d1 between a patch A of the first wedge and a patch B of a reference wedge, with d1 a specific colour distance that is based on q1. As discussed further below, the reference wedge may represent a standard state of the device; in another case, the reference wedge may be the first wedge itself, so that patches A and B both belong to the first wedge. Preferably, magnitudes of d1 are determined for several pairs of patches. The magnitudes of the colour distance between these patches are then used in calibrating the printing device.
Preferably, the printing device is a multicolour output device.
Two or more different wedges are printed; the wedges may be different in that a type of marking particles is used in printing the second wedge that is not used in printing the first wedge. A magnitude of a second quantity q2 is determined for at least one and preferably for each patch of the second wedge. The second quantity q2 is different from the first quantity q1; the magnitude(s) of q2 are also used in calibrating the printing device. The same preferences, mentioned for q1, apply for q2: preferably the second quantity q2 is a psychophysical quantity, more preferably a psychovisual quantity, most preferably a high quality psychovisual quantity; moreover, the second quantity is preferably determined with respect to a reference value which is the magnitude of q2 for the receiving substrate itself.
If the first wedge is a cyan wedge, a magenta wedge or a grey wedge, where a grey wedge is printed using black marking particles, the first quantity q1 preferably indicates brightness. For a yellow wedge, the determined quantity preferably indicates brightness and more preferably colourfulness. CIE lightness L* is a quantity that indicates brightness while CIE chroma C* indicates colourfulness; some other quantities indicating brightness or colourfulness may be found in [Hunt], section 7.2.
In a particular embodiment, a grey wedge is printed and a magnitude of the density is determined for at least one and preferably for each patch of the grey wedge.
An advantage of the invention is that a psychophysical or psychovisual quantity is determined, so that characteristics of the human visual system may be incorporated into the calibration method. An advantage of using a psychovisual quantity is related to the spacing of different colour shades over the available tonal range, which is discussed further below. An advantage of printing two or more different wedges and determining magnitudes of different quantities for the first and for the second wedge is that the choice of the quantities that are determined for the colorants of the printing device may be optimised for a given purpose. For one or more colorants, a psychophysical or a psychovisual quantity is determined, so that characteristics of the human visual system may be incorporated into the calibration method where they are desired. For other colorants, calibration may be based on density measurements or dot gains.
The invention may be applied to printing devices that use one type of marking particles per colorant. The invention may also be applied to printing devices using, for at least one colorant, two or more types of marking particles per colorant, e.g. light cyan marking particles and dark cyan marking particles as described above. In the latter case, the invention may be applied to a first kind of wedges that are printed to determine a LUT that is applied after a mixing table, such as LUT""s 42 and 43 in FIG. 5, discussed above under single ink calibration; the invention may be applied to a second kind of wedges that are printed to determine a LUT that is applied before a mixing table, such as LUT""s 45 to 48 in FIG. 4, discussed above, that may pertain to a mixture of several types of marking particles; the invention may be applied to both the first and the second kind of wedges.
Preferably, to calibrate a printing device that uses, for one or more specific colorants, two or more types of marking particles per colorant, magnitudes of a psychophysical or psychovisual quantity are determined for one or more and most preferably for all wedges of the specific colorants.
The invention may also be applied to wedges that are printed in determining a screening dependent LUT such as LUT""s 66 to 69 in FIG. 5, discussed above. The invention may also be applied to a combination of these wedges, e.g. to a wedge printed to determine LUT 46 and another wedge printed to determine LUT 42, or to wedges printed to determine respectively LUT""s 47 and 66, etc.
To summarise, the invention may be applied to any wedges of which magnitudes of a quantity are determined to be used anywhere in the calibration process of the printing device.
In a calibration method according to the invention, one or more magnitudes of a psychophysical or psychovisual quantity are determined as discussed above, but these magnitudes may be used in various ways in calibrating the printing device. Moreover, either the determined magnitudes may be used directly to calibrate the device, or a transformation, as known in the art of colour management, may be applied to the determined magnitudes before they are used in calibrating the device.
Determining, for at least one patch of a wedge, a magnitude of a quantity, may be carried out in different ways. The quantity, such as lightness or chroma, may be determined directly by means of a measuring instrumentxe2x80x94e.g. a colorimeter such as X-Rite""s model 948 (available from X-Rite, Grandville, Mich., USA) measures the amount of red, green and blue light reflected from a patch; this calorimeter uses CIE XYZ as the reference colour space and then converts the measured calorimetric data from CIE XYZ into CIE L*a*b* co-ordinates. Alternatively, a spectrophotometer may be used and the measured spectral data may be transformed into the desired quantity with a few calculations. Since a spectrophotometer is a very expensive instrument, in another embodiment a human observer visually determines the quantity, e.g. by visually comparing the concerned patch with a set of standard colour patches, such as patches from the Munsell colour atlas, of which the magnitude of the concerned quantity is known; this method of working takes time but is inexpensive and quite accurate, since the human eye is very sensitive to colour differences. In yet another embodiment, densities are measured using a densitometer that is fitted with suitable filters. Instead of directly using the measured densities or dot gains for calibration, as in traditional calibration methods, this embodiment according to the invention transforms the measured densities to magnitudes of quantities that include a psychophysical or psychovisual quantity. The transformation may be carried out by software and may include characteristics of the human visual system and models of the filters that are used in the densitometer and of the reflection propertiesxe2x80x94or the transmission propertiesxe2x80x94of the printed image. Such a model may then reconstruct spectral data from the measured density values, the characteristics of the used filters and the characteristics of the image printed on the receiving substrate. The accuracy of the reconstructed spectral data depends for a large part on the way in which the marking particles are applied to the receiving substrate to make up the printed image. The accuracy may be very high if different types of marking particles are on top of each other on the receiving substrate (such as in a colour photo that contains successive layers of dyes); the accuracy is much lower if the marking particles are applied next to each other, as in traditional offset printing. Moreover, each step in the transformation usually introduces an extra inaccuracy. Thus, for better accuracy, it is generally advantageous to determine psychophysical or psychovisual quantities in a direct way, e.g. by using a spectrophotometer or a calorimeter, instead of determining them via densities and a transformation. In still another embodiment, a CCD camera is used instead of a densitometer; the CCD camera may be equipped with filters, preferably with three RGB filters, and a transformation may be used to transform the assessed RGB values into e.g. CIELAB values, which are values of psychovisual quantities.
A system in accordance with the invention is claimed in claim 9. The printer, or printing device, may be a multicolour output device. The first and second quantifiers may either be the same quantifier (in which case both the first and the second magnitudes are determined by this same quantifier) or they may be different (e.g. the first quantifier may comprise a calorimeter and the second one may comprise a densitometer instead). The first and second calibrators may either be the same calibrator or they may be different. Preferably, the first calibrator or the second calibrator, more preferably both calibrators comprise calibration curves or LUT""s such as calibration curves 45 to 48 and 42 to 43 and LUT""s 66 to 69 discussed above.
To improve readability, xe2x80x9cthe quantifierxe2x80x9d in the description below indicates the first or the second quantifier or both quantifiers, while xe2x80x9cthe calibratorxe2x80x9d indicates the first or the second calibrator or both calibrators.
The quantifier may comprise a measuring instrument, such as a densitometer, or, which is preferred, a calorimeter or a spectrophotometer, and the quantifier may comprise a transforming device, preferably a calculating device, to transform one or more quantities, that were determined in a previous step, into quantities that are used for the calibration. The quantifier and the calibrator are provided by a system that comprises the printing device and that may comprise other devices such as a measuring instrument, a calculating device such as a computer, etc. The quantifier may be provided in many different ways by the devices that constitute the system, as is clear to those skilled in the art: the printing device may comprise a measuring instrument and a transforming device; the transforming device may be provided by a separate computer; magnitudes of a specific quantity may be determined in the printing device while magnitudes of another quantity may be determined by a separate measuring device; etc.
Preferably, the system also comprises a transmitter for transmitting signals from the quantifier to the printing device, and more preferably also from the printing device to the quantifier. In this way, the determined magnitudes may be transmitted from the quantifier to the printing device, e.g. by means of electrical or optical signals, by wire or wireless such as by electromagnetic waves. The determined magnitudes, or values based thereon, may then be stored in a memory in the printing device.
In a preferred embodiment, the system includes a mover for moving the receptor support during printing and the mover has a gripper for gripping the receptor support during the moving operation. The gripper may be a mechanical gripper including e.g. one or more rollers, sprocket wheels, friction wheels; any gripper as known in the art may be used. Preferably, the receiving substrate is still being gripped by the gripper during operation of the quantifier, i.e. when the magnitudes are determined for the patches of the wedge or wedges of the calibration target. This xe2x80x98mechanical couplingxe2x80x99 of the quantifier to the printing device via the receiving substrate and the gripper is discussed more in detail below, after the discussion of an important advantage.
Increased user convenience is an important advantage of the embodiment that has a transmitter for transmitting signals and especially of the embodiment wherein the quantifier is xe2x80x98mechanically coupledxe2x80x99 to the printing device via the receiving substrate and the gripper. In the latter case, calibration may only require that the printing device is provided with a receiving substrate and is instructed to calibrate itselfxe2x80x94the calibration procedure may then be performed automatically, i.e. without intervention from the user. On the other hand, a lot of intervention from the user is required in case the quantifier is not xe2x80x98mechanically coupledxe2x80x99 to the printing device via the receiving substrate and the gripper. Suppose for example that a separate measuring instrument is to be used. In such a case, the user first has to instruct the printing device to print a calibration target, then he has to transfer the receiving substrate on which the calibration target is printed from the printing device to the measuring instrument, subsequently he has to instruct the measuring instrument to do the desired measurements or he may even have to move the patches that are to be measured manually through the measuring instrument so that the next patch may be measured, then he has to transfer the determined magnitudes, which may be obtained from these measurements either directly or by an intermediate step such as a transformation on a specific calculating device, to the printing device, etc.
The xe2x80x98mechanical couplingxe2x80x99 of the quantifier to the printing device via the receiving substrate and the gripper may be realised in different ways. The mover, that includes the gripper, may be part of the printing device. An example is a printing device that has a transport mechanism that transports separate sheets of receiving substrate through the printing device. The mover may also be a device that is separated from the printing device; in case the receiving substrate is supplied to the printing device as a continuous web, the mover may include a supply spool that provides the web to the printing device, an uptake spool that receives the printed web, and a transport mechanism that transports the web through the printing device. In a first embodiment, the mover moves the receiving substrate, via the gripper, xe2x80x98throughxe2x80x99 the quantifier during operation of the quantifier. In a second embodiment, the quantifier has its own, additional mover so that the receiving substrate is moved with respect to the quantifier by the additional mover of the quantifier. Remark: the receiving substrate is preferably moved with respect to the quantifier in order to determine the magnitude for the next patch; what is important is that the movement is relative, i.e. the receiving substrate may also remain in its position while the quantifier is moved. Many other embodiments may be envisioned by the person skilled in the art to realise the xe2x80x98mechanical couplingxe2x80x99; it is important that the xe2x80x98mechanical couplingxe2x80x99 allows operation of the quantifier without requiring the user to transport the receiving substrate manually from the printing device to the quantifier.
Preferred embodiments of a system in accordance with the invention may include features of a methodxe2x80x94as claimed or as described above or belowxe2x80x94in accordance with the invention; e.g., in an embodiment of a system in accordance with the invention, the first quantifier is for determining a psychophysical quantity, preferably a psychovisual quantity, more preferably a high quality psychovisual quantity; e.g., for a yellow wedge, the quantity of which magnitudes are determined preferably indicates brightness and more preferably colourfulness.